Orthogonal Frequency Division Multiplexing (OFDM) has a high spectral efficiency (the spectrum of the subcarriers overlap) and combats frequency-selective fading. However, the amplitude of each carrier is affected by the Rayleigh law, hence flat fading occurs. Therefore, good channel estimation with an appropriate detection algorithm and channel coding is essential to compensate for fading.
The performance of OFDM frequency diversity is comparable to the performance of an optimal direct-sequence CDMA (DS-CDMA) system's multipath diversity (which requires a Rake receiver). Because diversity is inherent in OFDM, it is much simpler to achieve than in an optimal DS-CDMA system. An OFDM system benefits from a lower-speed parallel type of signal processing. A Rake receiver in an optimal DS-CDMA system uses a fast serial type of signal processing, which results in greater power consumption. In addition, the OFDM technique simplifies the channel estimation problem, thus simplifying the receiver design.
In multicarrier CDMA (MC-CDMA), a spreading sequence is converted from serial to parallel. Each chip in the sequence modulates a different carrier frequency. Thus, the resulting signal has a PN-coded structure in the frequency domain, and the processing gain is equal to the number of carriers. Some implementations of MC-CDMA employ frequency division multiple access. In multi-tone CDMA, the available spectrum is divided into a number of equal-width frequency bands that are used to transmit a narrowband direct-sequence waveform.
U.S. Pat. Nos. 5,519,692 and 5,563,906 describe geometric harmonic modulation (GHM) in which preamble and traffic waveforms are created from multiple carrier frequencies (tones) that are geometric harmonics of a fundamental tone. The waveforms incorporate binary-phase spreading codes, which are applied to the tones.
A preamble carrier waveform is constructed by summing binary-coded tones. Therefore, the preamble signals are similar to MC-CDMA signals. However, unlike MC-CDMA signals, the coded preamble tones do not carry data. Furthermore, due to the binary-phase coding, no tone phase alignment is provided that is capable of conveying data symbols on orthogonal tone superpositions. The inability of binary-phase (e.g., MC-CDMA) coding to produce orthogonal pulse waveforms is described in U.S. Pat. No. 5,955,992. In GHM, each receiver monitors the preamble signals for its assigned phase code and then decodes the appended traffic waveforms.
The traffic waveforms are products, rather than sums, of the binary phase coded tones. Thus, even when the tones are phase aligned, such as illustrated in FIGS. 1a, 1b, and 1c in the '906 patent, the resulting waveforms are not orthogonal pulse waveforms. The receiver generates a reference waveform from a product of tones having phase offsets corresponding to the receiver's phase code. The reference waveform is correlated with the received signals to produce a correlation result that is integrated over the data-bit duration and over all tones.
GHM does not employ polyphase codes. Thus, GHM does not provide carriers with phase relationships that enable precise time-domain control of the carrier superpositions. GHM does not produce pulse waveforms. Accordingly, GHM is not capable of expressing orthogonality in the time domain or reducing peak-to-average-power ratio (PAPR) problems associated with other multi-carrier protocols. Similarly, GHM is not capable of forming waveforms that are backwards compatibility with single-carrier protocols or other multi-carrier protocols.
U.S. Pat. No. 4,628,517 shows a radio system that modulates an information signal onto multiple carrier frequencies. No spreading codes are applied to the information signal or the carriers. Received carriers are each converted to the same intermediate frequency using a bank of conversion oscillators. The received signals are then summed to achieve the benefits of frequency diversity. In this case, frequency diversity is achieved at the expense of reduced bandwidth efficiency.
Each communications protocol presents different benefits and disadvantages. Benefits can be increased by merging different protocols, but only to a limited degree. There is a need for a protocol that solves all or most problems, and is adaptable to all conventional multicarrier and single carrier transmission protocols.